1. Field of the Invention
The invention relates to a method of displaying an image on a display device in a plurality of periods called sub-fields, wherein the display device is capable of generating in each of the sub-fields, a respective illumination level, the method comprising the steps of:
defining a set of combinations of sub-fields, each combination in the set corresponding with a respective illumination level of the display device;
for each pixel of the image, selecting from the set a particular combination of sub-fields, in conformance with the intensity value of the pixel; and
for each pixel of the image sending a representation of the selected combination of sub-fields to the display device for displaying the particular pixel.
The invention further relates to an image display unit for displaying an image on a display device in a plurality of periods called sub-fields, wherein the display device is capable of generating, in each of the sub-fields, a respective illumination level, the image display unit comprising:
storing means for storing a set of combinations of sub-fields, each combination in the set corresponding with a respective illumination level of the display device;
selection means for selecting from the set, a particular combination of sub-fields in conformance with the intensity value of a particular pixel of the image, and
sending means for sending a representation of the selected combination of sub-fields to the display device for displaying the particular pixel.
The invention further relates to an image display apparatus comprising such an image display unit.
2. Description of the Related Art
U.S. Pat. No. 5,841,413 describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds with a pixel (picture element) of the image that is to be displayed on the panel. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on, is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Typically, different sub-fields have a different duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32, respectively. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, whereby a code word indicates the intensity level of a pixel in binary form.
The device described in U.S. Pat. No. 5,841,413 employs more sub-fields than necessary for realizing the required set of intensity values. The resulting set of code words for expressing the intensity value is redundant, i.e. for a given intensity value, more than one code word is available. From this redundant set a subset is created whereby those code words are selected that give the fewest differences in the most significant bit for expressing a difference between the intensity values. This subset is created by searching the original set and determining what the effect on the artifacts may be for a difference between a given code word and each of the other code words. The resulting sub-set contains all different intensity values that can be generated by the combinations of the available sub-fields.
It is an object of the invention to provide a method as described in the preamble with an improved processing of the intensity values. This object is achieved, according to the invention, in a method which is characterized in that the combinations of sub-fields in the set correspond with respective illumination levels that are uniformly spaced on a perceptual scale. By choosing the working set of intensity levels, which is the set of intensity levels that are available for displaying the image, in such a way that the intensity levels are uniformly spaced with regard to human perception, the number of different intensity levels may be substantially reduced while still being able to display the image with sufficient quality. This is caused by the fact that the sub-field driven display, like a plasma panel, has a linear relation between the video input and the luminance output, while a human has a perception curve that strongly differs from a linear relation. The human viewer is very susceptible for differences for low intensities and less susceptible differences for high intensity. Therefore, near the dark end of the scale, a relatively large number of intensity levels are required having small mutual luminance differences, whereas near the bright end of the scale, fewer intensity levels are required which may have a large mutual luminance differences. The known display device has intensity levels with fixed luminance differences between them. This results in a luminance resolution that is more finely spaced than necessary for the high intensity levels. The method according to the invention provides a relatively large number of different intensity levels for low intensity and a relatively small number of levels for high intensity. In total, the number of intensity levels according to the invention is much lower than the number of intensity levels according to the known method. This is advantageous since fewer levels need to be stored for processing the image and less computational effort is required.
Above, it has been described that the selection and distribution according to the invention allows a reduction of the number of intensity levels and that this reduction was realized by making a selection among all levels possible in the display device at hand. However, it is also possible to exploit the reduced number of required intensity levels by reducing the number of levels that the device can generate, e.g. by modifying the organization of the sub-fields.
In an embodiment of the method according to the invention wherein the respective illumination levels have a luminance substantially according to the function L=xxcex3, wherein L is the output luminance, x is the number of the illumination level and xcex3 is a constant, such a function provides an easy approximation of a perceptual intensity scale.
In an embodiment of the method according to the invention wherein the value of xcex3 is approximately 2.3, this distribution of intensity levels corresponds to the inverse of the gamma filtering that is applied to video signals taken by a camera. When the levels of this distribution are directly used on the basis of the received video signals, the required gamma correction is intrinsically applied. Therefore, this embodiment does not require the separate step of inverse gamma filtering as applied in the known method.
In an embodiment of the method according to the invention wherein the combinations of sub-fields in the set are formed by sub-fields that are temporally close together compared to the total time of the plurality of sub-fields, by restricting the intensity levels that are used for displaying the image to those levels that are realized in sub-fields that follow each other in a short period of time, motion artifacts as described above are reduced. These artifacts may occur since in driving the display device, the frame period, i.e. the period between two successive images, is separated into a number of sub-field. During each of these sub-field periods a cell may or may not be switched on and integration over the sub-field periods results in a perceived intensity level of the pixel corresponding with this cell. Instead of displaying a pixel at a given moment in time, on a plasma display panel, the pixel is displayed as a series of sub-pixels shifted in time with respect to each other. This may cause artifacts if a series of images contains a moving object. The eyes of the viewer track the moving object, while the elements of the object emit light at various different moments. These temporal differences between parts of the object are translated to spatial differences by the eye tracking, resulting in artifacts like false contours. Another artifact is motion blur. This occurs if the intensity level of the pixels of a moving object are generated in a large number of sub-fields. It is then clearly noticeable that the light of a pixel has been emitted at the various different moments.
The motion of an object needs to be taken into account when displaying the object in a number of sub-fields. For each next sub-field, the object must be moved a little. Motion compensation techniques are used to calculate a corrected position for the sub-pixels in the sub-fields. In some circumstances, the motion compensation are not fully reliable and may produce erroneous results, e.g., in an area of the image with little detail. The erroneous results lead to motion compensation where this should not be done. This also gives motion artifacts which are very visible.
Now, in this embodiment, the light of a single pixel is emitted in a shorter period of time, which makes any possible motion during the emission shorter. Therefore, the light of a pixel is perceived to come from a single position, or at least from positions close together, thus resulting in a better picture with a smaller chance for artifacts.
In an embodiment of the method according to the invention wherein the combinations of sub-fields in the set are formed by 1 or 2 sub-fields that are temporally adjacent, by using only combinations of 1 or 2 sub-fields that are adjacent in time, it is possible to generate a suitable working set of illumination levels. This working set contains a sufficient number of illumination levels on the one hand, while on the other hand it is less sensitive to motion artifacts.
It is a further object of the invention to provide an image display unit as described in the preamble with an improved processing of the intensity values. This object is achieved according to the invention in an image display unit which is characterized in that the combinations of sub-fields in the set correspond with respective illumination levels that are uniformly spaced on a perceptual scale. These combinations allow the display of the image with a smaller number of different intensity levels, while maintaining the perceived quality of the image. This is caused by the fact that the distribution of the intensity levels according to the invention are adapted to human perception. The reduction of the number of intensity levels that needs to be employed is advantageous in view of storage and computational effort.